In industrial measurement technology, especially also in connection with the control and monitoring of automated manufacturing processes, for ascertaining characteristic measured variables of media, for example, liquids and/or gases, flowing in a process line, for example, a pipeline, often measuring systems are used, which by means of a measuring transducer of the vibration type and, connected thereto, driving, and evaluating, electronics (most often accommodated in a separate electronics housing) induce reaction forces, for example, Coriolis forces, in the flowing medium and produce, derived from these, a measurement signal correspondingly representing the at least one measured variable, for example, mass flow, density, viscosity or some other process parameter. Measuring systems of this kind, which are often formed by means of an inline measuring device in compact construction with integrated measuring transducer, such as, for instance, a Coriolis mass flow meter, have been known for a long time and have proven themselves in industrial use. Examples of such measuring systems having a measuring transducer of vibration type, or also individual components thereof, are described e.g. in WO-A 88/02475, WO-A 88/03642, WO-A 99 40 394, WO-A 08/059,015, WO-A 08/013,545, WO-A 07/043,996, WO-A 01 02 816, WO-A 00/12971, WO-A 00 14 485, U.S. Pat. Nos. 7,392,709, 7,360,451, 7,340,964, 7,299,699, 7,213,469, 7,080,564, 7,077,014, 7,073,396, 7,040,179, 7,017,424, 6,920,798, 6,883,387, 6,860,158, 6,840,109, 6,758,102, 6,691,583, 6,666,098, 6,651,513, 6,557,422, 6,519,828, 6,397,685, 6,378,364, 6,330,832, 6,223,605, 6,168,069, 7,337,676, 6,092,429, 6,047,457, 5,979,246, 5,945,609, 5,796,012, 5,796,011, 5,796,010, 5,731,527, 5,691,485, 5,610,342, 5,602,345, 5,531,126, 5,476,013, 5,398,554, 5,301,557, 5,291,792, 5,287,754, 4,823,614, 4,777,833, 4,738,144, US-A 20080250871, US-A 20080223150, or US-A 20080223149, US-A 2008/0141789, US-A 2008/0047361, US-A 2007/0186685, US-A 2007/0151371, US-A 2007/0151370, US-A 2007/0119265, US-A 2007/0119264, US-A 2006/0201260, U.S. Pat. No. 6,311,136, JP-A 9-015015, JP-A 8-136311, EP-A 317 340 or the not pre-published German patent application 102007062397.8. Each of the therein illustrated, measuring transducers comprises at least one, essentially straight, or at least one, curved, measuring tube for conveying the medium, which can, in given cases, also be extremely cold or extremely hot. Furthermore, each of the measuring transducers shown in U.S. Pat. Nos. 5,291,792, 5,945,609, 7,077,014, US-A 2007/0119264, WO-A 01 02 816 or also WO-A 99 40 394 includes a supplemental transducer housing (especially a supplemental transducer housing mounted directly to the inlet tube piece and to the outlet tube piece), which surrounds the measuring tube and the counteroscillator coupled thereto, as well as the exciter mechanism and sensor arrangement, while, for example, in the measuring transducer shown in U.S. Pat. No. 4,823,614, the transducer housing is quasi composed of the counteroscillator, or, in other words, transducer housing and counteroscillator are one and the same structural unit.
During operation of the measuring system, the at least one measuring tube is caused to vibrate for the purpose of generating oscillatory forms influenced by the medium flowing through the tube. For exciting oscillations of the at least one measuring tube, measuring transducers of the vibration type additionally include an exciter mechanism actuated by an appropriately conditioned, electric, driver signal, e.g. a controlled current and/or a controlled voltage, generated by the mentioned driver electronics. This excites the measuring tube to bending oscillations in the wanted mode by means of at least one electro-mechanical, especially electrodynamic, oscillation exciter, through which excitation current flows during operation. Furthermore, such measuring transducers include a sensor arrangement with oscillation sensors, especially electrodynamic oscillation sensors, for at least pointwise registering of oscillations, especially oscillations in the Coriolis mode, at the inlet, and outlet, sides of the at least one measuring tube and for producing electric sensor signals influenced by the process parameters to be registered, such as mass flow or density. In addition to the oscillation sensors provided for registering vibrations of the measuring tube, the measuring transducer can have still more sensors, as also provided in EP-A 831 306, U.S. Pat. Nos. 5,736,653, 5,381,697, or WO-A 01/02 816, among others, especially serving to register rather secondary measurement variables, such as e.g. temperature, acceleration, expansion, stress, etc., and arranged on, or in the vicinity of, the inner part formed, in any case, of measuring tube, counteroscillator, as well as the exciter mechanism and sensor arrangement attached thereto.
As excited oscillation form—the so-called wanted mode—in the case of measuring transducers with a curved, e.g. U, V, or Ω shaped measuring tube, normally the eigenoscillation form is selected, in which the measuring tube moves like a pendulum at least partially at a lowest natural resonance frequency about a longitudinal axis of the measuring transducer, in the manner of a cantilever fixed at an end, as a result of which mass flow dependent, Coriolis forces are induced in the medium flowing through the measuring tube. These in turn lead to the fact that, superimposed on the excited oscillations of the wanted mode, in the case of curved measuring tubes, thus pendulum-like cantilever oscillations, are bending oscillations of the same frequency corresponding to at least one, also natural, second oscillation form, the so-called Coriolis mode. In the case of measuring transducers with curved measuring tubes, these cantilever oscillations, caused by Coriolis forces, correspond usually with that eigenoscillation form in which the measuring tube also executes rotational oscillations about a vertical axis oriented perpendicular to the longitudinal axis. In the case of measuring transducers with straight measuring tubes, for the purpose of generating mass flow dependent, Coriolis forces, often a wanted mode is selected in which the measuring tube at least partially executes bending oscillations essentially in a single plane of oscillation, such that the oscillations in the Coriolis mode are formed, accordingly, as bending oscillations coplanar with the oscillations of the wanted mode, and are of the same oscillation frequency. As a result of the superimposing of wanted mode and Coriolis mode, the oscillations of the vibrating measuring tube registered by the sensor arrangement at the inlet and outlet sides of the measuring tube have a mass flow dependent, measurable, phase difference. Normally, the measuring tubes of such measuring transducers, e.g. those used in Coriolis mass flow meters, are excited during operation at an instantaneous, natural resonance frequency of the oscillation form selected for the wanted mode, especially at oscillation amplitude controlled to be constant. Since this resonance frequency especially is also dependent on the instantaneous density of the medium, commercially available Coriolis mass flow meters can measure, in addition to mass flow, also the density of media flowing in the measuring tube. Furthermore, it is also possible, as shown for example in U.S. Pat. Nos. 6,651,513 or 7,080,564, using measuring transducers of the vibration type, to directly measure the viscosity of the medium flowing through the measuring tube, for example based on an exciter power required for exciting the oscillations. In the case of measuring transducers with two measuring tubes, these are normally linked into the process line via a distributor piece on the inlet side, extending between the measuring tubes and a connecting flange on the inlet side, as well as via a distributor piece on the outlet side, extending between the measuring tubes and a connecting flange on the outlet side.
In the case of measuring transducers having a single measuring tube, such normally communicates with the process line via an essentially straight piece of connecting tube which opens into the inlet side of the measuring tube, as well as an essentially straight piece of connecting tube which opens into the outlet side of the measuring tube. Furthermore, each of the illustrated measuring transducers having a single measuring tube includes, composed of a single piece or multiple parts, at least one tubular, box-shaped, or plate-shaped counteroscillator, which, with formation of a first coupling zone, is coupled to the inlet side of the measuring tube, and, with formation of a second coupling zone, is coupled to the outlet side of the measuring tube, and which in operation essentially rests or oscillates equally and oppositely to the measuring tube, that is, with the same frequency and opposite phase. The inner part of the measuring transducer, formed by measuring tube and counteroscillator, is normally held in a protective, measuring transducer housing alone by means of the two pieces of connecting tube, via which the measuring tube communicates with the process line during operation, especially in a way enabling oscillation of the inner part relative to the measuring tube. In the case of measuring transducers shown in, for example, U.S. Pat. Nos. 5,291,792, 5,796,010, 5,945,609, 7,077,014, US-A 2007/0119264, WO-A 01 02 816, or also WO-A 99 40 394, having a single, essentially straight, measuring tube, the latter and the counteroscillator are oriented essentially coaxially to one another, as is common in conventional measuring transducers. In standard measuring transducers of the previously named type, the counteroscillator normally is also essentially tubular, and is formed as an essentially straight hollow cylinder, which is arranged in the measuring transducer such that the measuring tube is at least partially surrounded by the counteroscillator. Used as materials for such counteroscillators are normally relatively cost-efficient types of steel, such as structural steel, or free-machining steel, especially when titanium, tantalum, or zirconium are used for the measuring tube.
The exciter mechanism of measuring transducers of the type being discussed normally has at least one, usually electrodynamic, magnet assembly, serving as oscillation exciter, and acting differentially on the at least one measuring tube, and the, in given cases, present, counteroscillator, or the, in given cases, present, other measuring tube, while the sensor arrangement includes a electrodynamic magnet assembly on the inlet side of the measuring tube, serving as an inlet-side oscillation sensor, as well as at least one magnet assembly on the outlet side of the measuring tube, of essentially the same construction, serving as an outlet-side oscillation sensor. Usually, at least the magnet assemblies serving as oscillation sensors are essentially of the same construction. Such magnet assemblies serving as oscillation transducers of standard measuring transducers of vibration type are formed by means of a magnetic coil (in the case of measuring transducers with one measuring tube and a counteroscillator coupled thereto, the coil is normally mounted on the latter), as well as by means of an elongated, especially rod-shaped, permanent magnet, which, serving as an armature, interacts with the at least one magnetic coil, especially plunging into the coil, and which is mounted correspondingly on the measuring tube to be vibrated. This has the advantage, for example, that, by means of the magnet assemblies, the oscillatory movements between the vibrating measuring tube and its counterpart, that is, the, in given cases, present counteroscillator or the, in given cases, present, other measuring tube, can be differentially registered, or produced, as the case may be. The permanent magnet and the magnetic coil serving as exciter, or sensor, coil are, in such case, normally oriented essentially coaxially to one another. Additionally, in the case of conventional measuring transducers, the magnet assembly serving as oscillation exciter is normally formed and positioned in the measuring transducer in such a way that it acts essentially centrally on the at least one measuring tube. In such case, the magnet assembly serving as oscillation exciter is, as shown, for example, also in the measuring transducers disclosed in U.S. Pat. Nos. 5,796,010, 6,840,109, 7,077,014 or 7,017,424, usually mounted at least pointwise along an imaginary central peripheral line of the measuring tube on its outer side. Alternatively to oscillation exciters formed by means of a magnet assembly acting centrally and directly on the measuring tube, exciter mechanisms formed, as provided in U.S. Pat. Nos. 6,557,422, 6,092,429 or 4,823,614 among others, for example, by means of two magnet assemblies mounted not in the center of the measuring tube, but, instead, shifted, respectively, toward its inlet and outlet sides, can also be used, or, as provided in U.S. Pat. Nos. 6,223,605 or 5,531,126, among others, exciter mechanisms formed, for example, by means of a magnet assembly working between the, in given cases, present counteroscillator and the measuring transducer housing, are also used.
In the case of measuring transducers of the type being discussed, it is, as mentioned also in U.S. Pat. Nos. 6,047,457 or 6,920,798, among others, common to connect magnetic coils and the corresponding permanent magnet of the magnet assemblies serving as oscillation transducers—it may be an oscillation exciter or an oscillation sensor—to ring- or washer-shaped, especially metal, mounting elements attached to the measuring tube. These mounting elements securely surround the measuring tube essentially along circumferential lines of the measuring tube. The particular mounting element, as provided in U.S. Pat. Nos. 6,047,457, 7,299,699, US-A 2006/0201260, U.S. Pat. Nos. 5,610,342, or 6,519,828, among others, can be fixed to the measuring tube by pressing externally, by hydraulic pressing or rolling from inside of the measuring tube, or by thermal shrink fit, especially in such a manner that it is lastingly subjected to elastic or mixed plastic-elastic deformations, and as a result, is permanently prestressed radially with respect to the measuring tube.
In standard measuring transducers of the vibration type, the magnet assemblies serving as oscillation sensors are, as already indicated, often essentially of the same construction as the at least one magnet assembly serving as the oscillation exciter, insofar as they work according to the same operating principle. Accordingly, the magnet assemblies of such a sensor arrangement are also mostly formed, in each case, of: at least one magnetic coil—normally fixed on a, in given cases, present counteroscillator—and, at least at times, passed through by a variable magnetic field, and thus at least periodically provided with an induced measurement voltage; as well as a rod-shaped permanent magnet, mounted on the measuring tube and interacting with the at least one magnetic coil, and providing the magnetic field. Additionally, each of the aforementioned coils is connected with the mentioned operating and evaluation electronics of the in-line measuring device by means of at least one pair of electric, connecting lines, which are normally run along the shortest route possible from the coils, over the counteroscillator, to the transducer housing.
In the case of magnet assemblies of the aforementioned type, for the purpose of homogenizing the magnetic field flowing through the coil and the permanent magnet, as well as for the purpose of avoiding disturbing stray fields, the permanent magnet is normally placed within a magnet cup composed at least partially of magnetically conductive material, and is secured to a cup base, from which extends an essentially tubular, especially circular cylindrically formed wall of the magnet cup. Normally, the permanent magnet is arranged essentially in a center of the cup base, and usually affixed to this such that permanent magnet and cup wall are oriented coaxially to one another.
For securing the permanent magnet and magnet cup in magnet assemblies of the type being discussed, as shown in WO-A 88/02475 for example, a clamp screw is fed through a bore provided in the permanent magnet, and through a corresponding bore in the cup base, and is tightened with an appropriate clamp nut. However, such a clamp screw—especially the screw-head, ultimately forming the free end of the permanent magnet—can undesirably deform, and, in this respect, disturb, the magnetic field carried in the magnet assembly.
To avoid such disturbances of the magnetic field, in magnet assemblies of standard measuring transducers of the vibration type, as also shown in WO-A 07/043,996 or WO-A 00/12971, among others, the permanent magnet and cup base are often connected with one another by a material bond, for instance by brazing or welding, if necessary also using a sleeve pressed onto the permanent magnet, and moderating between the permanent magnet and the braze material. Furthermore, it is quite common to fix the permanent magnet to the cup base using an adhesive bond. However, as also mentioned in WO-A 00/12971 or U.S. Pat. No. 6,883,387 among others, magnet assemblies of the type being discussed can be exposed to significant stresses resulting from very high (>200° C.) or very low (<−50° C.) operating temperatures, and/or resulting from high acceleration forces (>10 G), such that the material bonds formed between the permanent magnet and cup base, either through adhesive or brazed connections, must fulfill very high quality requirements, especially with regard to fatigue strength under operating conditions.
A disadvantage of such connections using material bonds between the permanent magnet and cup base, however, is that especially also due to the mounting position of the permanent magnet within the magnet cup, as well as the very small dimensions of the permanent magnet and magnet cup, the application of the substances ultimately forming the material bonds, for instance the braze material or the adhesive, on the one hand, and, on the other hand, the highly precise orientation of the permanent magnet within the magnet cup, can be related to considerable difficulties, and in that respect can be very complicated. Additionally, due to the most often very different materials for the permanent magnet and cup base, especially with regard to workability and required fatigue strength over a broad thermal and/or mechanical stress range, really well-suited braze material or adhesive is not readily available, or else is very expensive.